564 



EXPLORATION GEOPHYSICS 



A good example of the use of electrical prospecting for locating a fault 

 zone is described by Hawley.f The complete survey consisted of two 

 parts, viz., a constant depth survey (utilizing constant electrode spacing) 

 and depth profiles (moving electrodes outwardly and symmetrically from 

 the fixed center). In both cases the Wenner configuration was used. The 

 comparison of results of the two parts and the problems in interpretation 

 make the article a good reference. 



Fig. 348. — Bedrock contours and typical subsurface profile, Santa Ynez River, 

 California. (.The Mining Journal, Jan. 15, 1931.) 



Civil Engineering Construction Problems. — Geophysical investiga- 

 tions are of especial value in the following engineering construction 

 problems: (1) examination and location of dam and reservoir sites; (2) 

 location of water wells; (3) examination and location of aqueduct and 

 pipe line routes; (4) location of rock and gravel quarries; and (5) tunnel 

 construction. 



The application of geoelectrical methods will be illustrated by des- 

 criptions of two representative surveys: (1) a survey conducted at the 

 Fort Peck Dam and Reservoir, Montana, and (2) a survey conducted in 

 the vicinity of the Gillespie Dam on the Gila River near Phoenix, Arizona. 



Fort Peck Dam 



The primary purposes of the Fort Peck geophysical investigations were 

 to determine: (1) the major structural features of the sedimentary rocks 

 underlying the dam and reservoir sites, with special reference to the loca- 



t Paul F. Hawley, "Fault Location by Electrical Prospecting — an Example," Geophysics, Vol. 

 VIII, No. 4, October, 1943, pp. 391-403. 



